Fish

There are nearly 40 native freshwater fish species in New Zealand, many of which are found in braided rivers and associated wetlands. Several websites are dedicated to freshwater fish, which are both important sources of food for braided river birds and crucial to the in-stream health of the rivers. The larvae of freshwater mussels, for example, which filter huge volumes of water, use our native fish to hitchhike to new locations. Changes in the physical character of braided rivers, the amount and type of vegetation along their banks, pollution and water abstraction, and the introduction of top-end in-stream predatory trout and salmon have all contributed to a decrease in numbers of native freshwater species. Some, like the Canterbury mudfish/kowara, are on the verge of extinction.

Stream food web (a) before and (b) after trout invasion. The size of an organism indicates the relative population biomass of that species (based on reported results from a field experiment conducted in the Shag River in central Otago; Flecker and Townsend 1994). The dashed line indicates a trophic cascade effect of trout on algae (Image: Elizabeth Graham in Waiology)

“Braided rivers and their associated wetlands are important habitat for several native fish. Two migratory and eight non-migratory native freshwater fish are found in rivers and streams of the Mackenzie Basin.

The non-migratory bignose galaxiid is found only in streams in this region. Two other endangered non-migratory species are also present – the nationally threatened lowland longjaw and upland longjaw galaxias. Lowland longjaws are unusual in that they are known to burrow into gravels and spawn underground. Other non-migratory fish include upland and common bullies, and alpine galaxias.

Longfin eel and kōaro (whitebait species) are the only native fish in the upper Waitaki that normally migrate to the sea to spawn. Longfin eel are also classified as threatened. They are a long-lived fish, which spawn once in their lifetime, and their numbers are declining due to the combined effects of commercial harvest and loss of habitat.

The protection of waterways inhabited by native fish in the upper Waitaki catchment is an important priority. We need to find out more about the ecology of non-migratory fish, in particular, so we can understand their requirements and likely responses to change. The increasing number of threats facing streams in the catchment (including resource consent applications to extract water and changes to water quality as land uses intensify) emphasises the urgency of this work.

Habitat requirement

Instream cover, such as vegetation, rocks and boulders, is a major habitat requirement of these fish. Larger fish spend most of their time concealed amongst instream cover, venturing out into the open only to hunt for food. These hunting forays may occur only at first light, in the evening or during the hours of darkness. This is how they grow so large, and are seldom seen or caught.

New Zealand native freshwater fish and insects have adapted to cooler waters, which have higher oxygen levels. Once water reaches 16° C the oxygen levels are significantly reduced. A vegetation canopy can provide cover and shading, which helps to regulate water temperature. Leaves and other debris provide habitat for a myriad of insects both aquatic and terrestrial, many of which either fall or fly onto the water surface. This provides an important food source for native fish species as well as sports fish and wading birds.”